1. Field of the Invention
The invention relates to a write signal control circuit in an optical disk drive, and more particularly to a write signal control circuit in an optical disk drive for generating write signals having proper duty cycles.
2. Description of the Related Art
FIG. 1 illustrates a block diagram of a laser power control system in a typical optical disk writer. Referring to FIG. 1, the laser power control system 10 includes an EFM (Eight-to-Fourteen Modulation) encoder 11, a write signal generator 12, a LD (Laser diode) driver 13, and a LD 14. The EFM encoder 11 receives the input data, and generates an EFM signal according to the EFM encoding rules. The write signal generator 12 receives the EFM signal and converts the EFM signal into a plurality of write signals, e.g. three write signals WS1, WS2, WS3 as shown in FIG. 1, according to the write strategy waveform generating rules. The write signal generator 12 typically includes a write strategy pulse generator 121, a flip-flop unit 122, and a write signal calculator 123. The LD driver 13 receives the plurality of write signals and integrates them into a drive signal to drive the LD 14. How the write signal generator 12 converts the EFM signal into the write signals according to the write strategy waveform generating rules is a well-known technology in this field and can be found in U.S. Pat. No. 6,445,661, and detailed description will be omitted.
Along with the increase in write speed of the optical disk drive, the timing accuracy among the write signals becomes more critical. Correspondingly, the problems of the waveform distortion caused by the digital logic gate, the buffers, and the output drive within the write signal generator also become more serious. One of the waveform distortions is regarding to the distorted duty cycle. FIG. 2A and FIG. 2B show the waveforms of three write signals with ideal duty cycles and the corresponding drive signal, wherein FIG. 2A shows three write signals of the write signal generator 12, and FIG. 2B shows the drive signal generated by the LD driver 13. Because the duty cycles of the three write signals WS1, WS2 and WS3 inputted to the LD driver 13 are ideal, the drive signal generated by the LD driver 13 is also ideal accordingly.
FIG. 3A and FIG. 3B show the deformed write signals and the corresponding drive signal outputted by the LD driver 13, wherein FIG. 3A shows the three write signals of the write signal generator 12, and FIG. 3B shows the drive signal generated by the LD driver 13. As shown in FIG. 3A and FIG. 3B, because the duty cycles of the three write signals WS1, WS2 and WS3 are distorted, the drive signal generated by the LD driver 13 is seriously deformed, as compared that shown in FIG. 2B. If the optical disk drive outputs such a deformed drive signal to the LD 14, the data may not be recorded in the optical disk in a proper format, thereby causing data error when one tries to reproduce the data in the optical disk. Therefore, it is an important subject to provide adjusted write signals having proper duty cycles.